Random Quadrat Sampling Method
for the Rocky Intertidal

Marine Introduced Species Monitoring Resource Center

OVERVIEW: Rocky intertidal shoreline sampling should be targeted at a cobble area of the shoreline in between the mid to low tide zones. Two of the targeted invasive species for the rocky intertidal habitats are Hemigrapsus sanguineus (Asian shore crab) and Carcinus maenas (European green crab). These crabs normally seek shelter between the cobble and rocks of the intertidal zone.

This monitoring must be conducted at or near low tide. A target evaluation area is selected and divided into four equal sections (quadrants). A square-meter quadrat is randomly placed in each of the four quadrants. It is important to understand that the evaluation area is consciously selected to allow for repeatability, but that the quadrat sampling is random. Random sampling means that every area within the evaluation area has an equal chance of being measured and that one quadrat placement does not determine or influence the next quadrat placement. Randomness eliminates bias and ensures that each subset is independent. If the location of the sampling quadrat is chosen in a nonrandom manner, an unrealistic assessment of the evaluation area will result.

It is a fallacy to think a person can go out in the field and place a quadrat randomly in the study area without bias. Unknowingly, the size of the rocks, the amount of water, the glimpse of a crab shell, or something else will influence the decision. This is haphazard sampling and studies have shown that haphazard placements result in non-random distributions with study area edges being underrepresented (Greig-Smith 1983). Data collected in a haphazard manner will be discounted as statistically unusable.

Each quadrat has its own data sheet on which all species present are recorded. Abundance estimates are used for some common and often prolific species such as common periwinkles (Littorina littorea). An actual count is taken for other invertebrates, such as the crabs, sea stars and sea urchins. Sessile organisms (seaweeds, mussels, barnacles) are given percent coverage estimates, while mobile invertebrates are either counted or given abundance estimates.

Monthly sampling during spring, summer, and fall is a good target. Monitoring once during the winter months would be a bonus, but weather conditions do not always permit this. Monitoring in teams of two to four people is recommended. The safety of the monitors is always of paramount importance, and people should never put themselves in danger to collect data. This method may be used for tide pools with one data sheet per monitoring session.


Two 30-meter tape measures or two ropes with meter markings
Clipboard and pencil
Four data sheets
Rulers with metric scale

Protective footwear

Marine Invasive ID Cards
Marine Field Guides
Ziplock bags or watertight containers
70% - 95% ethanol or isopropyl alcohol
Net for mobile organisms

DIRECTIONS How to set-up your monitoring site:

1. Establish a fixed evaluation area between the mid and low tide lines by locating the largest and best suitable area of rocky or cobble habitat in a particular region.

2. Tide charts are available at www.maineharbors.com/ma/tidemans.htm.

3. Divide the area into four quadrants (sections) by placing a meter tape (knotted rope) parallel to the water's edge for a distance of 20 meters and perpendicular to waterline for 20 meters. The two tapes must cross each other at the mid-intersection, in this case, at 10 meters. Ideal set-up is 20 meters by 20 meters If available area is narrower than 20 meters If area is shallower than 20 meters.

4. At the initial setup, map the area and key landmarks for repeatability. Ascertain the site coordinates using a GPS or from a topographic map.


1. Return to your chosen evaluation site within one to two hours of low tide.

2. Record the names of all monitors present, date, time, site name, time of low tide, weather conditions, air and water temperature, and salinity using a refractometer.

3. Choose a quadrant near the low water edge to begin sampling.

4. Place the quadrat using two random numbers generated from a random number table. A random number table is provided.

5. Starting in the center of the plot, walk parallel to the water, marking off on the x-axis the first random number. Then, walk perpendicular to the water, marking off the second random number on the y-axis.

6. Place a 1-meter2 quadrat at the intersection of these two random numbers (x, y). The quadrat should be placed on the ground inside of (x, y) as shown in the diagram. Placement of quadrat within a quadrant.

7. Using the data sheet, record all organisms in the quadrat.

8. If you want to know more about crabs than their numbers, place crabs in a bucket as you find them. For more on measuring and determining whether a crab is a male or female.

9. Data keeper: Record percent coverage of sessile organisms (invertebrates like mussels, barnacles, and all types of algae) and abundance (averages and actual counts) for mobile invertebrates (e.g., crabs, snails, sea stars).

10. After visible organisms have been counted, turn over rocks and pebbles to look for crabs. Tunicates, bryozoans, snails, and mollusks may also be observed on the underside of rocks.

11. Sample one quadrat in each quadrant (NE, NW, SE, SW) following the same random number procedure.

12. Even though the intertidal zone experiences regular disturbances from wave exposure, temperature and other human and animal presence, care should be taken to avoid creating disturbances or undo stress that may affect settlement and/or life processes of the organisms.

13. Photograph or sketch rare invaders or unusual observations e.g. Didemnum sp.

14. Return data sheets to the coordinating organization for data entry.

For Percent Coverage Diagram,
Determining Randomness and Safety, download
A Citizen's Guide to Monitoring Marine Invasive Species

QUADRAT DATA SHEET - 2 pages( front/back PDF)
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